In addition to customer's payload signals such as voice, data or other forms of information, a subscriber's loop circuit carry various control signals which are exchanged between a CPE and a switch at a CO. These control signals (simply called local signal or signaling) are generated according to a widely accepted signaling protocol and perform functions such as, ringing, detecting the off-hook, on-hook or line-busy conditions and so on. Recently much more functions are added to those existing ones in order to provide new types of customer services. These new services include call answer, call waiting, call forward, and caller identification to name a few.
CLASS (custom local area signaling services) is a signaling protocol currently being widely accepted by the industry to provide this wide range of services. This will be described in detail below as an example. Control data format in CLASS is serial, binary and asynchronous. The modulation is voiceband FSK, using 1200 and 2200 Hz for logical 1 and 0 respectively. FIG. 1 shows fields of the data format of CLASS. Each data word consists of an 8-bit byte, each data word is preceded by a start bit (space) and followed by a stop bit (mark), a total of 10 bits in a word. Transmission of any 8-bit character is supported in a message word. The alert signal consists of 180 bits (in on-hook transmission) and 80 bits (in off-hook transmission) of continuous mark. The channel seizure signal is present during on-hook transmission only. It consists of 300 continuous bits of alternating space and mark, beginning with space and ending with mark. The transmission rate is 1200 Baud.
SCWID (Spontaneous Call Waiting InDication) or Call Waiting Caller ID or off Hook Caller ID is a technique of signaling to the subscriber the identity of a call waiting caller while the subscriber is off-hook on another call. The subscriber can then choose to talk to the call waiting caller by performing a hook flash or "Link". This feature is one of many services which are envisaged and are being implemented by using CLASS. In this application due to lack of a proper terminology, SCWID is used throughout to indicate these features which use off-hook signaling. U.S. Pat. No. 5,263,084 issued on Nov. 16, 1993 to Chaput et al. describes the operation of one typical off-hook signaling technology.
Briefly the protocol is as follows: When a caller is waiting, the office switch sends a SAS (subscriber alert signal) tone and then a CAS (custom alert signal) tone in voiceband. The subscriber apparatus detects the CAS tone and if it is capable of receiving further CLASS or other information it returns to the switch an ACK (acknowledge) signal (a DTMF tone). The switch then sends the FSK data packet containing the call waiting caller's identity or any other information in the data format described above. During the time from the CAS to the end of the FSK data packet the subscriber apparatus mutes the RX and TX paths to subscriber so that they will not hear or possibly corrupt the data transport mechanisms.
BellCore, who set forth a CLASS procedure, recommends that SCWID equipped subscriber apparatus (telephone set) should check if there are any off-hook extensions on the loop before sending back the ACK to the switch after reception of a CAS. If the switch does not receive an ACK from the subscriber apparatus within a predetermined period of time (e.g., maximum of 100ms), it will not send FSK data containing the call wailing caller ID. In the known arrangement, SCWID equipped subscriber apparatus and adjuncts monitor the DC conditions on the loop and if an extension is detected then they do not send an ACK to the switch. This means that call waiting caller ID or off-hook caller ID or other off-hook signaling does not work if an extension is off-hook.
It should be noted that while the description which will follow deals primarily SCWID (off-hook caller ID), FSK signals which are exchanged between the terminal and office switch after the ACK signal can be used to transmit different information in addition to the caller ID for a variety of other services.